The invention relates to a sensor assembly for determining an angular position of a rotor relative to a stator according to the preamble of claim 1, and also relates to a method for determining an angular position of a rotor relative to a stator using a sensor assembly.
Generic sensor assemblies are used to determine the angular position of a rotor relative to a stator in a non-contact manner. Generally, the rotor and the stator are components which are rotatable relative to each other about an axis of rotation. For example, the rotor may be a shaft and the stator may be a bearing housing. For example, the rotor may be adapted to rotate about the axis of rotation while the stator is stationary, so that the rotor can rotate about the axis of rotation relative to the stator. For example, both the stator and the rotor may be adapted to rotate about the same axis of rotation, and the rotor may be rotatable relative to the stator about the axis of rotation. The angular position of the rotor relative to the stator is defined by an angle about which the rotor is rotated about the axis of rotation relative to the stator. Thus, generic sensor assemblies are configured so that first an initial angular position of the rotor relative to the stator can be determined, and then the angle at which the rotor is located relative to the stator can be continuously detected starting from the initial angular position, and the angle can be indicated unambiguously between 0 and 2π.
Generic sensor assemblies have two components, namely a magnet and a sensor, where one of the components can be arranged and fixed on the rotor and the other one on the stator, for non-contact detection of the angular position of the rotor relative to the stator. The magnet is configured and arranged relative to the sensor in such a way that it generates a magnetic field at the sensor, which field varies as a function of the angle defining the angular position about the axis of rotation, and is distributed periodically with a period length of 2π or an integer fraction of 2π. For an unambiguous determination of the angular position, the sensor usually has a plurality of sensor elements, each of which is configured to determine the magnetic field strength to which it is exposed, and the exact angular position is obtained by comparison measurements of the different sensor elements of the sensor. In this case, the sensor elements of the sensor are arranged at different positions relative to the magnet, so that they each measure a different magnetic field strength in dependence of the position of the magnet, i.e. its angle about the axis of rotation, so that a comparison measurement, i.e. a comparison of the different measured values derived from different sensor elements, theoretically allows for a particularly precise determination of the angular position using the sensor assembly.
Various implementations of the magnet and the sensor of generic sensor assemblies are known from the prior art. For example, multi-pole ring magnets are usually used as the magnets (see e.g. the sensor assemblies of WO 2014/029885 A1 or DE 11 2007 003 276 T5), since such magnets are capable of providing, in a simple manner, a magnetic field with a magnetic field strength that varies periodically, with short period lengths, as a function of the angle about the axis of rotation. This is particularly advantageous for a most precise possible determination of the angle, and thus, when used on the rotor and stator, of the angular position of the rotor relative to the stator. In conventional sensor assemblies, the magnet is usually arranged on the end face of the rotor, that is on a face of the rotor through which the axis of rotation extends, and the sensor is arranged along the axis of rotation slightly spaced apart from the magnet and the end face of the rotor, so that the sensor is capable of detecting particularly well the magnetic field and accordingly also the variation of the magnetic field as a function of the angle of the magnet about the axis of rotation. However, a fundamental problem of conventional sensor assemblies is that at the locations of application of the sensor assemblies, the presence of external magnetic fields (interfering magnetic fields) cannot be excluded, so that the determination of the angular position by comparison measurements using a plurality of sensor elements is often incorrect, since the external magnetic field has different external magnetic field strengths at different sensor elements, so that the different measured values of the magnetic field strength derived from different sensor elements are not only caused by the position of the magnet of the sensor assembly relative to the respective sensor element but also by the external magnetic field.
In the sensor assembly according to WO 2014/029885 A1, this problem is addressed by having the sensor of the sensor assembly arranged along the axis of rotation facing a magnet in the form of a multi-pole ring magnet and within the radial extent of this magnet, characterized in that the sensor has two sensor pairs each comprising two sensor elements, all the sensor elements are arranged on a circle about the axis of rotation within the radial extent of the ring magnet, and the sensor elements of a sensor pair are arranged diametrically opposite to each other relative to the axis of rotation and symmetrically to the axis. All the sensor elements are adapted to determine the field component of the magnetic field provided by the magnet along the axis of rotation, and the sensor elements of a sensor pair that are arranged opposite to each other are arranged in opposite orientations to each other. By summing the measured values of the sensor elements of a sensor pair it is therefore possible to average out an external magnetic field under the assumption that the external magnetic field is the same on the two sensor elements. The prerequisite for an error-free functionality of the described sensor assembly is always that the sensor elements of a sensor pair are arranged as close to each other as possible and within the radial extent of the ring magnet, since only in this case it is possible to reliably average out an external magnetic field by summation of the measured values of the sensor elements of the sensor pair, while at the same time determining the angle of the magnet about the axis of rotation without error, because of the angle-dependence of the magnetic field which is given in this case. The sensor elements can then be arranged within a radial range around the axis of rotation in which the magnetic field strength changes monotonously along the axis of rotation, so that a reliable determination of the angle of the magnet about the axis of rotation is enabled even in case of a slight shift in location of the sensor relative to the magnet. Hence, a particular drawback of the described sensor assemblies is that the sensor assemblies necessarily have to be arranged on the end face of a rotor, so that an error-free determination of the angular position can be accomplished. It is not possible, for example, to arrange the sensor assembly at a large radial distance from the axis of rotation, for example on the radially outer surface of a rotor.